1. Technical Field
This invention relates generally to input and output scanners, and more particularly to a scanner having a multifaceted rotating polygon that directs a beam of light through a post-facet lens system toward a surface to be scanned.
2. Background Information
Parent application Ser. No. 496,459 filed Mar. 20, 1990, of which this application is a continuation-in-part, is incorporated by reference for the scanner background information and details of scanner construction provided. It describes a scanner with a post-facet lens system that compensates for field curvature and wobble without compensating for scanner non-linearity. That results in the post-facet lens system being less complicated. It is less costly. It avoids the introduction of unwanted aberrations. It can be configured with just two elements, and it still allows non-optical compensation for scanner non-linearity if desired.
Despite those advantages, manufacturers seek additional cost reductions. For that purpose, plastic elements might seem a likely choice because plastic elements can cost far less than glass elements. But using plastic involves problems such as the indeterminate surface tension of hot plastic. It can cause flat surfaces to deform during cooling and thereby complicate the manufacturing process.
In addition, plastic exhibits greater changes in refractive index with temperature than does glass and that can change the location of the focus beyond acceptable limits. In that regard, the toroidal wobble-compensating element in many existing scanner designs causes the cross-scan plane to be several times more sensitive to temperature defocusing than the scan plane. That is because the toroidal shape magnifies the image of the facet at the photoreceptor.
So, existing post-facet lens systems do not use plastic elements despite a potential for significant cost reduction. Therefore, it is desirable to have some way to overcome the problems described.